The present invention relates to processes and apparatuses for the anaerobic digestion of organic matter, and more particularly to an orbicular vessel having two adjoining chambers for the separate treatment of high solids and low solids phases of organic matter.
Anaerobic digestion is the process by which microorganisms in an oxygen-free environment transform organic materials into biogas, nutrients, and additional cell matter, leaving salts and refractory organic matter. This process produces a source of energy while reducing the pollution potential of the waste. The biogas produced during this process is a mixture of carbon dioxide and methane (the principal component of natural gas) and may be used as a fuel. This technology is used to treat a wide variety of solid and liquid waste streams. The primary alternatives to this technology are aerobic waste water treatment for liquid waste streams and composting for solid waste streams. Anaerobic processes have advantages over aerobic waste water treatment and composting of solid waste such as reduced mixing requirements, no aeration requirements, energy production, less sludge accumulation, and lack of odor emissions.
After anaerobic treatment, most waste water is quite amenable to land application for recovery of fertilizer nutrients which are conserved by the process. Discharge of the waste to surface waters however requires further treatment to remove residual organics and nutrients. Solid residues can be land applied in a manner similar to compost.
Compared to other waste treatment technologies that require significant energy inputs, anaerobic digestion is a net producer of energy with sufficient energy produced to power the waste treatment process and meet additional energy needs. Furthermore, because anaerobic systems are air tight, the chance for odor and other gaseous pollutant emissions are negligible in comparison to aerobic processes. This also holds true for liquid emissions that are often a problem with composting technologies. Anaerobic digestion also contributes to reduction of greenhouse gas emissions. Unlike burning fossil fuels, use of waste renewable resources represents a closed carbon cycle and thus does not contribute to increases in atmospheric concentration of carbon dioxide. Anaerobic digestion can also remove heavy metals from waste streams by precipitation reactions and dechlorinate chlorinated organics.
The ability of the anaerobic process to remove organics with a minimum of sludge production and aeration demand continues to spur the development of new reactor designs applied to a variety of waste streams. Applications of the anaerobic digestion process can generally be divided into low-solids wastewater pretreatment systems and slurry or high solids systems. The slurry and high solids systems typically operate in a batch, fed-batch, or intermittently-fed mode, while low solids wastewater pretreatment systems are normally continuously fed and operated at a higher flow rate.
Recently, anaerobic wastewater pretreatment has enjoyed extensive acceptance for a variety of industrial waste waters associated with food processing, beverages, breweries, distilleries, and most recently pulp and paper production. Lagoons are the most basic application of anaerobic digestion to waste, yet they do not readily accommodate recovery of biogas. In the simplest reactor design, a continuously-fed continuously-stirred reactor (CSTR), the liquid waste is pumped through a heated tank. With this design the slow growth of the microorganisms must be compensated for by using large tanks which allow for the high hydraulic retention times (HRT=volume of digester/flow rate) of 10 to 20 days to avoid washout of the microorganisms in a CSTR. To correct for this deficiency, a number of digester designs have been developed in the prior art for various waste types. McCarty (1982) demonstrated that waste treatment in a reactor in which effluent solids are recycled is dependent on sludge age or solids retention time (SRT) rather than HRT as in a CSTR. [McCarty, P. L., (1982). One Hundred Years of Anaerobic Treatment. Anaerobic Digestion, 1981. Ed: D. E. Hughes, et al., Elsevier Biomedical Press B.V., Amsterdam, 3–21.] This discovery allowed him to develop the “anaerobic filter” which, like an aerobic trickling filter, uses a stationary support material to provide surface for growth of bacteria and retain them in the reactor. Lettinga (1978) developed the up flow anaerobic sludge blanket (UASB) reactor as a modification to the anaerobic filter. [Lettinga, G., (1978). Feasibility of anaerobic digestion for the purification of industrial wastewater, Proc. 4th European Sewage and Refuse Symp., Munich]. Van den Berg developed a hybrid anaerobic filter/sludge bed reactor. These reactors allow a HRT as low as 1 hr. [Van den Berg, L., (1985). The downflow fixed film and upflow blanket filter reactor. Anaerobic Digestion 1985, Proc. 4th Int. Symp. on An. Dig., Nov. 11–15, 1985, China State Biogas Association, Guangzhou, China.]
A number of designs that treat solid wastes have also been developed. These designs generally employ “cement mixer” or percolating bed approaches as exemplified in the following references: Ghosh, U.S. Pat. No. 4,396,402; Chynoweth, U.S. Pat. No. 5,269,634; Zhang, U.S. Pat. No. 6,342,378.
A digester system described by Gosh (U.S. Pat. No. 4,022,665) describes a method for the biochemical separation of anaerobic digestion into an initial acid forming or hydrolysis step and a second step in which the byproducts of this step are converted to methane. Subsequent improvements on this process (Ghosh et al U.S. Pat. No. 4,396,402, and Ghosh et al, U.S. Pat. No. 4,696,746) involved the separation of the two phases using separate vessels. U.S. Pat. No. 4,396,402 describes a leaching bed system to percolate liquid through a bed of solid waste to entrain solubulized degradation products that are then conveyed to a separate reaction vessel containing a microbial population that converts these products to methane and carbon dioxide. A portion of the treated liquid is then recycled to the leach bed to entrain further hydrolysis products. This system is similar to a system developed by Zhang et al. (U.S. Pat. No. 6,342,378) in which solid waste is leached in one vessel and the resulting leachate is processed in a second vessel. Another approach to high solids digestion employs multiple, hydraulically linked, leaching beds, as shown by Chynoweth and LeGrand in U.S. Pat. No. 5,269,634. Hall (1988) developed a three-stage system in which straw and dairy manure were treated in three vessels, each having a different maturity. [Hall, S., A. Thomas, F. Hawkes, D. Hawkes, (1988). Operation of Linked Percolating Packed Bed AnaerobicDigesters. In: Fifth International Symposium in Anaerobic Digestion, Bologna, Italy (1988)]. The first stage of the Hall system is a newly loaded primary reactor in which leachate is percolated through a mixture of wheat straw and dairy manure. Percolate from the primary reactor is pumped to a second reactor. Percolate from the second reactor is pumped to a third reactor, where enough methanogens have accumulated to convert the leached acids into biogas. Finally, percolate from the third reactor is pumped to the primary reactor in order to leach out more acids and inoculate the feed with methanogens. This process was adapted for the treatment of municipal solid waste (MSW) by Chynoweth and LeGrand (U.S. Pat. No. 5,269,634) in which liquid leachate is pumped between high solids leach beds of differing maturities. The operation of these multiple stage, batch reactors requires critical leachate management and complicated recirculation schemes to avoid process upset or “pickling” of the digesters through build up of organic acids and the resulting inhibition of methane formation. As is known to those skilled in the art, these systems require complex plumbing, pump systems, and controls or vigilant operator supervision to operate effectively. Furthermore, linked batch leach bed systems are only effective at treating wastes that have sufficiently recalcitrant components needed to provide physical support for biofilm production in the mature stages. Highly degradable wastes, such as food waste, will completely degrade leaving no physical support for biofilm production. This eliminates the potential for maintaining “mature batches” required to operate linked batch reactors. This limits this type of reactor to the treatment of wastes containing plastics or poorly degradable forms of paper, and plant materials such as municipal solid waste (MSW), manures containing substantial amounts of bedding, or woody and high lignin containing plant materials.
Several anaerobic digesters employ multi-baffled vessels with multiple chambers in a linear array within a single vessel (see Srinivasanand Sansalone, U.S. Patent Application 2003/0034300 A1, Shih U.S. Pat. No. 5,525,229). The ability of these digesters to treat high solids waste is dependent on the settling out of the high solids components in the first chamber and the passing of a low solids leachate through the remainder of the various chambers. However, as is known to those skilled in the art, most high solids wastes, including MSW, food waste, and manures, have some components that sink and others that float. As a result, these multi-baffled, serpentine designs can quickly become clogged with solids if used for the treatment of most high solids waste streams.
What is needed then is a simple anaerobic digester that is not susceptible to clogging and provides for separation, and independent treatment, of solid and liquid components of waste streams.